1. Field of the Invention
The present invention relates to the semiconductor field, and particularly relates to a method for forming a gate electrode.
2. Description of Prior Art
In conventional technology, processes for forming gate electrode can be divided into processes for gate-first and processes for gate-last. A process for gate-first includes steps: firstly, depositing a gate dielectric layer; forming a gate electrode on the gate dielectric layer; then performing ion implantation to form source and drain; and perform annealing to activate ions in the source and drain. The process for gate-first is simple, but the gate electrode inevitably suffers from high temperature which causes a drift of the threshold voltage Vt of the MOS transistor and affects the performance of the MOS transistor. A process for gate-last includes steps: after annealing, which is after the step of high temperature, etching a polysilicon dummy gate to form dummy gate trench; then filling the dummy gate trench to form gate electrode with proper metal material. Such process for gate-last prevents the gate electrode from high temperature, which avoids the drift of the threshold voltage Vt of the MOS transistor.
The process for gate-last, which allows a wide range of material for the gate electrode, is complicated. With the continued scaling-down of semiconductor device dimensions, during forming the metal gate electrode, the filling efficiency of metal material can not achieve 100% because of the decreased width of the dummy gate trench, especially in processes of 32 nm and below. That is to say there are holes in the metal material filling the dummy gate trench, which increase the parasitic resistance of the gate electrode and decrease the reliability of the MOS transistor.
A conventional method for forming a metal gate includes: providing a substrate; forming a dummy gate electrode structure on the semiconductor substrate, the dummy gate electrode structure including polysilicon; removing the dummy gate electrode structure to provide a trench with a top and a bottom, the top and the bottom having a first width; increasing the width of the top, the top having a second width; and forming a gate electrode in the trench, which includes a step of depositing a first metal in the trench. Such method for forming a gate electrode includes a step of increasing the width of the top of the trench after removing the dummy gate electrode structure, which facilitates the filling of metal into the trench, thus improving the filling efficiency of metal material. However, the Argon sputtering process which is used in the method to increase the width of the top of the trench tends to cause damage to the substrate.